Literally -- humanism from the latin studia humanitatis -- or studies of the human mind and form. Humanism was denoted as such (the study of man) to distinguish itself from the earlier forms of scholasticism -- or the study of God and the works of God, eg: the bible, etc. Humanism advocated the intellectual development of man without direction from God, through secular learning and the study of classical literature. It is precisely the thoughts of humanism which tipped off the later Reformation movements. See Humanists and the Reformation for opinions on this.
Humanism began its rise amongst the writers of the 14th century in Italy, and sparked off a movement that became the Renaissance. The earliest humanists were authors such as Boccaccio (1313-75) who composed the Decameron -- the first modern, secular short story collection. Another 14th century humanist was the poet Petrarch (1304-74), who translated and studied the works of Cicero and promoted the secular learning of the early Greeks.
The study of humanism began by the focus on the three forms of the Trivium -- grammar, rhetoric, and logic, as well as the study of poetics, in particular the classical. Initially these forms were all studied from the classical, and in particular the Greek world -- the movement of Byzantine scholars to Italy from around 1400 where they taught the Greek language enabled scholars to study classical Greek texts in their original language, and to perform many translations.
Study of the classical Greek (and hence pagan) writers had been, prior to humanism not been popular. For example, the scholastics of the 12th century and 13th century had become too absorbed in theology according to the humanists, and therefore overlooked the wisdom of the ancients -- this was the greatest criticism of the scholastics by the humanists. As a counter to this, humanists taught ethics rather than theology.
Because the Greeks, and thus the humanists had on their side many of the great philosophers, which the scholastics chose to ignore, the humanists saw the scholastics as having a rigid, formalistic, aged and unproper way of doing philosophy.
This rise in the study of Greek and other (eg: Latin) secular texts during the 15th century gave Humanism the ability to edge-out its competitive form, scholasticism in the universities. Important secular humanists from Italy such as Ficino, Pico della Mirandola and Lorenzo Valla promoted humanism as a world-wide movement. The first humanist pope, Nicholas V was the founder of Rome's Vatican Library, one of the world's most important (at the time) collections of humanist literature.
By the end of the 15th century, humanism, and in its wake the Renaissance had become established throughout the main centers of Europe. One of the world's most important humanists of his day, Erasmus (1469? - 1536), was born in Holland and travelled widely throughout Europe, including Italy and England. One of Erasmus' great works, The Praise of Folly was translated into almost every language, including English, by the end of the 17th century.